The mayor said earlier this week that he wants the commission to start working out how it will conduct its interviews, while acknowledging that it would be difficult if the officer under investigation is chosen by it as their pick. The mayor has final say over who gets the job. Public Safety Commission Chairman William J. Fennell could not be reached for comment.

Police Capt. Daniel M. O’Brien has been under investigation by the U.S. Attorney’s Office since February over his treatment of a woman in custody during the Big E last fall, according to the mayor.

Late last month, the mayor said that he had spoken to the federal office five weeks previously and had been told the probe ought to wrap up in “a couple of months.

Meanwhile, Christine A. Sterling, spokeswoman for the U.S. Attorney’s Office, said Wednesday she could neither confirm nor deny the existence of an investigation of O’Brien.

The mayor has said it is difficult to evaluate the three top candidates because of the federal investigation and that many people believe that probe should have wrapped.

Neffinger had noted that Hampden District Attorney Mark G. Mastroianni announced in February that he had completed the portion of the investigation his office will do and had turned over the results to the U.S. Attorney’s Office in Springfield.

The city has been without a permanent police chief since former longtime Police Chief Thomas E. Burke retired in March. Police Capt. Ronald P. Campurciani is running the department in the meantime. Campurciani and Capt. Daniel M. Spaulding are the other finalists for the job.

Consultants Mark Morse Consulting Group of Brookline selected the three finalists earlier this summer following an assessment-style evaluation.

O’Brien has been on paid administrative leave from the Police Department pending the conclusion of the investigation. O’Brien declined to comment on the situation Wednesday.